Biological thiols such as Cysteine (Cys), Homocysteine (Hcy) and Glutathione (GSH) play crucial roles in maintaining cellular antioxidant defence system. Among them cysteine plays many important roles in living systems. Cysteine is one of the three main precursors required for GSH synthesis. The deficiency of this compound causes many diseases such as slowed growth in children, depigmentation of hair, edema, liver damage, skin lesions, and weakness. An elevated level of Hcy is a risk factor for cardio-vascular disease, dementia and Alzheimer's disease. Abnormal levels of GSH is connected to many diseases such as HIV, cell death and aging. Thus, detection and discrimination of these thiol containing molecules are of great importance. Because of the similar structure and reactivity, distinction among biothiols is a challenging task. The respective concentration level of Cys and His in human plasma is typically 240-360 μM and 15-75 μM. Estimation of these amino thiols in human blood plasma is essential for understanding the role of these in the pathogenesis of vascular diseases.

The past two decade has seen significant effort being devoted to the development of optical probes for the selective recognition of thiol containing amino acids. There are many strategies for sensing biothiols, based on Micheal addition, cyclization with aldehydes, disulfide cleavage and others.
Das et al. (Chem. commun., 2014, 50, 9899-9902) discloses chemo dosimetric reagents for biothiol. A Cu (II)-complex based probe for detection of Cys and Histidine was reported.
P Das et al. (Org. Biomol. Chem., 2013, 11, 6604-6614) reports a new and simple chemodosimetric probe L11 is utilized for the selective detection of biothiols in the presence of other relevant amino acids under physiological conditions (pH=7.4). Furthermore, the studies with human blood plasma demonstrated the possibility of using this reagent for the quantitative optical detection of total biothiols in biological fluid.
X Yang et al. (Angewandte Chemie International Edition, Nov. 4, 2011, Volume 50, Issue 45, pages 10690-10693) reports a benzothiazole derivative used to detect cysteine (Cys) and homocysteine (Hcy) simultaneously in neutral media. The method involves thioether formation followed by cyclization to release 2-(2′-hydroxy-3′-methoxyphenyl) benzothiazole (HMBT) and a lactam.
P Das et al. (Chemistry—A European Journal, Nov. 26, 2012, Volume 18, Issue 48, pages 15382-15393) reports rationally designed and synthesized two new reagents L1 {(4-(1H-imidazo[4,5-f][1,10]phenanthrolin-2-yl)benzaldehyde)} and L2 {(4-(6,11-dioxo-6,11-dihydro-3H-anthra[1,2-d]imidazol-2-yl)benzaldehyde)}, each bearing a pendant aldehyde functionality. This aldehyde group can take part in cyclization reactions with 13- or γ-amino thiols to yield the corresponding thiazolidine and thiazinane derivatives, respectively. These two chemodosimetric reagents could be used for the quantitative detection of cysteine present in blood plasma by using a pre-column HPLC technique.
However, the development of probes for specific discrimination of biothiols is an unmet need in the art. Probes that are selective to any one of these amino biothiols are very rare in literature. Probes that give specific response with colour change as well as emission change are much needed. Especially those probes which allows real time monitoring without the aid of any instrumental techniques are highly recommended as for as the practical utility is concerned. There are no reports on detection of thiols using simple test strips. Particularly, a further challenge that is unaddressed in the art is that of a simple process for the determination of free cysteine.
Therefore, there is need in the art to develop a reagent for selective detection of free cysteine, especially with a high degree of selectivity towards free cysteine. Accordingly, the inventors of present invention developed a novel ligand for the selective detection of free cysteine.